Increasing restrictions on the amount and types of volatiles which may be released in work environments and the desire to reduce energy consumption have prompted the development of radiation curable coating compositions which are essentially free of volatile solvents that must be evaporated during the curing of the composition. These solvent-free coating compositions are known as 100 percent reactive systems; that is, each component of the composition reacts and becomes incorporated into the cured coating when the composition is exposed to radiation.
The radiation curable coating compositions of the prior art typically contain a radiation reactive oligomer or resin, a radiation reactive diluent, a photoinitiator and, optionally a radiation reactive crosslinker. The radiation reactive diluent serves the function of reducing the viscosity of the oligomer or resin in order that the composition, in the uncured state, has a viscosity such that it can easily be applied as a film to a substrate using conventional techniques of the coatings art.
Virtually any monomer or oligomer which can be polymerized by a conventional thermally initiated polymerization reaction can be employed as one of the radiation reactive components of the radiation curable coating compositions of the prior art. However, compounds containing acrylyl or methacrylyl groups have become by far the most widely used components of radiation curable coating compositions because of the ease and rapidity with which the acrylyl or methacrylyl groups undergo radiation-induced addition polymerization. Monofunctional monomeric acrylate or methacrylate esters are generally employed as the radiation reactive diluent; monomeric polyfunctional acrylate or methacrylate esters are employed as the crosslinking agent; and oligomers or resins containing one or more acrylyl or methacrylyl groups are employed as the oligomer or resin component.
While the aforementioned acrylyl and methacrylyl bearing compounds are excellent in terms of their radiation responsiveness, and they produce cured coatings having good physical properties, their use in radiation curable coating compositions has disadvantages related to their generally high levels of toxicity. Special handling techniques generally must be used to prevent workers from coming in contact with these materials or their vapors. This is particularly true of the monofunctional monomeric acrylate or methacrylate diluent, since it is usually the lowest molecular weight component and the most likely component to produce vapors. Typical monofunctional monomeric acrylate or methacrylate diluents include 2-hydroxyethyl acrylate, methyl acrylate, hexyl acrylate, 2-hydroxypropyl acrylate, glycidyl acrylate, 2-phenoxy acrylate, 2-methoxyethyl acrylate, 2-ethylhexyl acrylate, isodecyl acrylate, 2-(N-methylcarbamoyloxy) ethyl acrylate, dicyclopentyl acrylate, and the like, or the corresponding methacrylates.
When the cured coatings are to be employed in applications where they are to come in contact with the skin, even a small amount of residual unreacted acrylyl or methacrylyl monomer being present in the cured coating is a hazard, since unreacted monomer can migrate out of the coating.
The radiation curable coatings industry has been searching for low toxicity radiation reactive components which can be employed in radiation curable coating compositions to reduce the degree of dependence on acrylic compounds. It would be especially desirable to find a suitable low toxicity substitute for the monofunctional monomeric acrylate or methacrylate diluent component. To date, no completely satisfactory substitute has been found. The lack of a suitable low-toxicity diluent for radiation curable coating compositions has been an impediment to their widespread use in textile coating applications.